> From: Damien Broderick <firstname.lastname@example.org>
> At 09:27 PM 7/07/99 +0200, Anders wrote:
> >It also shows that nearby supernovae aren't necessarily
> >planetkillers. As far as I know there was no major extinction the last
> >million years.
This seems correct. The atmosphere functions as a pretty good shield. However, if you wait long enough one will occur close enough to you that the atmosphere doesn't help much. [That is a good reason why SIs aren't located in close proximity to us unless they are actively defusing the larger nearby stars. But more on that later...]
The current theories on Supernovas, I believe has them forming as a result of gravitational waves triggering large gas cloud compressions and bursts of large star formations. These rapidly evolve into supernovas. There was something about the rotation of the galactic arms causing such waves/star formation bursts/SNovas at periodic intervals. So your neighborhood may be comfortable for quite a long period of time and then get really nasty.
There is also the theory that the Sun's orbit crossing through the galactic plane causes the disruptions in the Oort cloud/Kuiper Belt that then results in increased bombardment for short periods.
Plus you have the problem of stars randomly orbiting close to our solar system causing orbital disruptions. Think about all those stars with those big planets in very strange orbits...
So in the "windows of opportunity" you need to evolve to the point where you can anticipate, plan for and survive these catastrophes.
> No, but if you're inside a relativistic starship (or time-machine, in my
> fanciful version in DREAMING DRAGONS), that's a lot of compressed sleet all
> at once... I get more and more worried about the hazard of travelling
> fast. Nano streams catapulted to near c, the front-runners taking the
> ablation shock (but maybe, alas, adding to it in secondaries), sounds like
> maybe the only plan that's safe against wear&tear.
There were several articles in JBIS regarding the problems of high velocity travel back when people seriously considered things like this. A simple little H-atom packs a heck of a lot of energy at .99c. The particles are difficult to deflect and really do a number on any nanomachinery. Sure you can stick a big Tungsten shield out front but that makes your starship/nanoprobe heavy and drives up your acceleration costs. Ignoring the fact that all that energy you spend to get to those high speeds could better have been spent "thinking" while you were going slow.
I believe Robert Freitas is planing to do a more in-depth analysis of the radiation damage problem on nanomachinery in Volume II of Nanomedicine (expanding significantly on Eric's rather brief treatment). Unfortunately we will have to wait a year or two to get it.